Percutaneous electrode array

ABSTRACT

A percutaneous electrode array is disclosed for applying therapeutic electrical energy to a treatment site in the body of a patient. The array comprises a plurality of electrode microstructures which are inserted into the epidermis, thereby overcoming the inherent electrical impedance of the outer skin layers and obviating the need to prepare the skin surface prior to an electro-therapy treatment. The array preferably includes an adhesion layer to help keep the electrode microstructures inserted into the epidermis during the duration of the therapeutic treatment, and temperature and condition monitoring devices to ensure proper treatment and enhance patient safety.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. application Ser. No. 09/756,999,filed Jan. 8, 2001, now U.S. Pat. No. 6,584,358, which claims priorityto U.S. provisional application No. 60/175,003, filed on Jan. 7, 2000and also to U.S. provisional application No. 60/183,258, filed on Feb.17, 2000, each of which is hereby incorporated by reference for each ofits teachings and embodiments.

FIELD OF THE INVENTION

This invention relates to an electro-therapy method and apparatus andmore particularly to a method and apparatus for applying a therapeuticelectrical signal for relieving pain arising from temporary or chronicconditions or during or after surgery.

BACKGROUND OF THE INVENTION

Electro-therapy is the application of electrical energy to the body of ahuman patient to provide a therapeutic effect. The therapeutic effectsproduced by electro-therapy include the blockage of pain, residual painrelief possibly due to the release of endorphins or other opiate-likeanalogs, relief from headache pain, increase of blood flow, increases inthe range of motion, cartilage regrowth or regeneration, acceleratedbone growth, electronic epidural for childbirth and other beneficialeffects that result from the introduction of a low frequency electricfield into tissue beneath the skin. Electro-therapy as defined by thisapplication does not include electro-osmosis, electroporation, oriontophoresis, or any other process in which electrical energy such asan electrical field or electric currents are used to promote thetransdermal transportation of chemicals or fluids into or out of thebody. Nor does it include electrosurgery where radiofrequency electricalenergy is used to cut or cauterize tissue.

Electro-therapy typically employs a non-invasive technique to introducethe electrical energy into the patient's body. Disposable electrode padsare placed on the epidermal surface of a patient and coupled to anelectric generator. The generator supplies two or more oscillating orcomplex morphology electric currents to a patient, with respectiveselected electrode pads separated from one another on the patient's bodywith a pain site located between the electrode pads with the majority ofthe electric field positioned perpendicular to each skin surface onwhich the pads reside. The electric currents have frequencies of atleast about 1 KHz and differing by as little as 1 Hz up to about 250 Hzfrom each other. A non-linear action of nerve fiber membranes and/orother electrochemically-active structures or fluids causes a mixing ofthe two independent frequency signals in a volume of tissue surroundingand beneath the pads along an axis between them to produce a therapeuticeffect. The mixing yields a distribution of synthesized sum anddifference frequencies among which is a therapeutic low frequencyequivalent to a beat frequency of the signals.

In order to penetrate the tissue beneath the skin and provide atherapeutic effect, electrical signals applied to the body must overcomethe electrical impedance of the skin. Electrical impedance is a propertyof the skin that limits the amount of current that can pass through theskin. The top layer of the skin, the stratum corneum, is made up of deadskin cells and contributes to the skin's high electrical impedance. Dry,intact skin can have an impedance which exceeds a hundred thousand ohms.Even carefully prepared skin, i.e., where the hair has been shaved orotherwise removed, where debridement of devitalized or contaminatedtissue has been performed, and where the skin's surface has beenmoisturized, can still have an impedance of over one thousand ohms. Apotentially large voltage would be necessary to overcome the skinimpedance and drive a therapeutically useful amount of electricalcurrent through body tissues. The relatively large amount of energyrequired limits the amount of time that a portable generator devicepowered by batteries can be used.

Additionally, electrical currents may travel across or just beneath thesurface of the skin, further reducing the amount of useful currentprovided to body tissues. This leakage current arises from the variouslayers of skin, and can limit the range of frequencies that can beapplied to body structures. The skin layers contribute electricalcapacitance and resistive properties which act as a barrier to currentflow, thus requiring a larger power source to compensate for the leakagecurrent, further limiting battery lifetime.

Biomedical studies conducted in other unrelated fields have determinedways to reduce skin impedance. For example, one study involved the useof a silicon micro-needle array to evaluate large-moleculetransportation properties of the array/skin interface (See Henry, S. etal., “Microfabricated Microneedles: A Novel Approach to Transdermal DrugDelivery,” 87 J. Pharm. Sci. 922–925 (1998)). A micro-needle array is anarray of small injection needles having a limited length so that asufficient quantity of drugs can be injected though the needles into theskin, without the accompanying pain perceived by the patient as with astandard injection needle. Volunteers described the sensation of amicro-needle array insertion as being similar to affixing a piece oftape to the skin. This study showed that the micro-needle array caused a50-fold drop in skin resistance.

In another study, an array of silver or silver with silver chloridecoated spikes were used as electrodes for electroencephalography (EEG),i.e., the measurement of electrical activity of the brain. (See Griss,P. et al., “Characterization of Micromachined Spiked BiopotentialElectrodes,” 49 IEEE Trans. Biomed. Eng. 597–604 (2002)). The array wasapplied to the forehead of the patient to monitor EEG activity. Thearray was used to overcome skin resistance in order to detect the weakEEG electrical signals produced by the brain.

In addition, patents have been granted for needle arrays used inconjunction with iontophoresis and electroporation. In iontophoresis,and electric field is used to accelerate ionized molecules for additionto or removal from the body. For example, Gartstein et al. disclose intheir U.S. Pat. No. 6,379,324 issued on Apr. 30, 2002 a molded or castplastic micro-needle array in combination with an anode and cathodeelectrodes. Ionized drugs are accelerated into the body due to theapplied electric potential. Additionally, the array uses an electricfield to remove fluid from the body for analysis by a biologicalelectrochemical sensor.

In electroporation, short pulses of high electric fields are applied tothe cells causing the cell wall to transiently become porous. Theapplied electric field is adjusted to ensure that permanent damage tothe cell wall does not result. Dev et al. disclose in their U.S. Pat.No. 6,451,002 issued on Sep. 17, 2002 a method for the treatment oftumors using an array of needles. High amplitude electrical signals areapplied to the needles that cause electroporation of the tissue cellsbetween the needles. Drugs used to treat the tumor are injected throughthe needles contemporaneously with the electroporation, therebyincreasing their introduction into the tissue cells.

Electrosurgery is the use of electrical radio frequency energy to cuttissue and coagulate bleeding during surgery. In such a procedure, theelectrical energy is delivered to the patient through a probe. The probepermits the physician to direct the electrical energy to the areas ofthe patient's body that she wishes to cut. In order to complete theelectrical circuit, a return electrode is applied to the patient. Thereturn electrode employs a large surface area contacting the patient toreduce the current density and prevent burning of the patient's skin atthe return electrode. For example, Fleenor et al. disclose in their U.S.Pat. No. 6,544,258 issued Apr. 8, 2003 a self-regulating andself-limiting electrosurgical return electrode pad. A patient lies downon top of the pad during an electrosurgical procedure. The pad has alarge surface area designed to prevent high current densities andtemperature rise, thereby preventing patient trauma.

Electrode pads designed for use with medical test procedures such aselectrocardiograms (ECGs) typically employ an electrical conductor, suchas a lead wire, electrically connected to an electrolyte disposed withinthe electrode pad. For example, Cartmell et al. discloses in their U.S.Pat. No. 4,699,679 issued on Oct. 13, 1987 a disposable medicalelectrode pad that includes two foam sheets with electrically conductiveadhesive layers on their lower surfaces. The pad further includes anelectrolyte gel matrix between the foam sheets. These pads are designedfor monitoring electrical signals produced by the patient, but aresometimes used to apply stimulation signals to a patent, such as inelectro-therapy.

It is known in the art that applying electrical energy to the skin canreduce the impedance of the skin. For example, Carim et al. discloses intheir U.S. Pat. No. 6,032,060 issued on Feb. 29, 2000 directingelectrical energy through a medical electrode placed on the skin of thepatient in order to electrically condition the skin. The reduction inskin impedance increases the ability to monitor bioelectric signals andcan reduce the amount of energy necessary for electroporation ortransdermal iontophoresis.

Each of the above references provide and devices are designed forsensing electrical signals generated by the body, for deliveringpharmaceuticals to the body, or for performing electrical surgery on thebody. These devices disclosed by the references have physicalcharacteristics and electrical properties which make them suitable fortheir intended uses; however, they are not designed for electro-therapy.

SUMMARY OF THE INVENTION

A percutaneous electrode array is disclosed for applying therapeuticelectrical energy to a treatment site in the body of a patient. Thearray comprises a plurality of electrode microstructures which areinserted into the epidermis, thereby overcoming the inherent electricalimpedance of the outer skin layers and obviating the need to prepare theskin surface prior to an electro-therapy treatment. The array preferablyincludes an adhesion layer to help keep the electrode microstructuresinserted into the epidermis during the duration of the therapeutictreatment, and temperature and condition monitoring devices to ensureproper treatment and enhance patient safety.

In one aspect, the present invention is directed to a percutaneouselectrode array for delivering therapeutic electrical energy to apatient, comprising: a substrate having a top side and a bottom side;and a plurality of electrodes each having a proximal end, a distal end,an axis from the proximal end to the distal end, and a length along theaxis, wherein each electrode is attached to the top side of thesubstrate; wherein the electrodes have a total surface area of more than0.2 square centimeters.

In another aspect of the present invention, the electrodes aresubstantially a cylinder and have a diameter of 20 to 250 micrometers.

In another aspect of the present invention, the electrodes aresubstantially a rectangular parallelepiped having a pair of narrowsides, a pair of wide sides, a top side and a bottom side, and whereinthe wide sides have a width of 20 to 250 micrometers.

In another aspect of the present invention, the wide sides have a widthof about 200 micrometers.

In another aspect of the present invention, the length of the electrodesis between 120 and 500 micrometers.

In another aspect of the present invention, the length of the electrodesis between 150 and 200 micrometers.

In another aspect of the present invention, the distal end of eachelectrode is one or more of thinned and pointed to facilitate placementinto skin.

In another aspect of the present invention, the axis of the electrodesis perpendicular to the substrate.

In another aspect of the present invention, the axis of the electrodesis angled between perpendicular and parallel to the substrate.

In another aspect of the present invention, the substrate comprises ashape-memory metal alloy.

In another aspect of the present invention, the electrodes comprise oneor more of doped semiconductor material, silicon-metal compound,stainless steel, conductive polymer, carbon allotrope, and a conductivemetal either in bulk or deposited material.

In another aspect of the present invention, a temperature element isbonded to the substrate.

In another aspect of the present invention, the temperature element isone of a thermistor, a diode, a semiconductor junction, and athermocouple.

In another aspect of the present invention, the array further comprisesan adhesion layer.

In another aspect of the present invention, the array further comprisesa plurality of voids that pass through the top side of the substrate tothe bottom side; and an adhesion layer comprising a bottom side, a topside, and a plurality of protrusions extending above the top side;wherein the top side of the adhesion layer is attached to the bottomside of the substrate, and the protrusions pass through the voids to afirst height above the top side of the substrate.

In another aspect of the present invention, the electrodes extend abovethe first height of the adhesion layer between 150 and 200 micrometers.

In another aspect of the present invention, the electrodes have a totalsurface area above the first height of the adhesion layer of at least0.2 square centimeters.

In another aspect of the present invention, the adhesion layer comprisesan electrically conductive hydrogel.

In another aspect of the present invention, the adhesion layer comprisesa removable medical adhesive.

In another aspect of the present invention, the adhesion layer changescolor as a function of ambient conditions.

In another aspect of the present invention, the array further comprisesa capacitive plate disposed on the bottom side of the adhesion layer andan electrically insulating layer disposed on the capacitive plateopposite the adhesion layer.

In another aspect of the present invention, the array further comprisesa temperature element embedded in the adhesion layer.

In another aspect, the present invention is directed to a percutaneouselectrode array for delivering therapeutic electrical energy to apatient, comprising: a substrate having a top side and a bottom side;and a plurality of electrodes each having a proximal end, a distal end,an axis from the proximal end to the distal end, and a length along theaxis, wherein each electrode is attached to the top side of thesubstrate, the substrate has a surface area of greater than 14.1 squaremillimeters and the electrodes have a total surface area of less than0.2 square centimeters.

In another aspect, the present invention is directed to an electrode fordelivering therapeutic electrical energy to a patient, comprising: asubstrate having a first side and a second side; an adhesion layercomprising a bottom side and a top side attached to the first side ofthe substrate; a capacitive plate disposed on the bottom side of theadhesion layer; and an electrically insulating layer disposed on thecapacitive plate opposite the adhesion layer.

In another aspect, the present invention is directed to an electrode fordelivering therapeutic electrical energy to a patient, comprising: asubstrate having a first side and a second side; and a temperatureelement bonded to the substrate.

In another aspect, the present invention is directed to a method ofproducing a percutaneous electrode array comprising: micromachining amaster mold of a percutaneous electrode array having a substrate and aplurality of electrodes from silicon using semiconductor lithographicprocessing; creating a replica mold by electroplating thin film silverfollowed by nickel onto the master mold; heating, softening, and rollinga polymeric film; forcing the film into the replica mold using pressureto form an array structure; and cooling the array structure and removingthe structure from the replica mold.

In another aspect of the present invention, the method further comprisesthermally processing array material to form a carbonized structure; anddepositing an adhesive layer on the structure.

In another aspect of the present invention, the polymeric film comprisespolymethyl methacrylate.

In another aspect of the present invention, the method further comprisesspraying conductive inks onto the structure and heating the structure toform a conductive coating.

In another aspect of the present invention, the method further comprisesspraying, dipping or spin coating an indium tin oxide precursor onto thearray structure; and heating the structure to form a conductive filmcoating.

In another aspect of the present invention, the method further comprisesforming a conductive film comprising indium tin oxide by evaporation orsputtering processes onto the array structure.

In another aspect, the present invention is directed to a method ofintroducing therapeutic electrical energy to body tissues in a treatmentsite beneath the epidermis of a patient, comprising: providing anelectro-therapy apparatus comprising: a signal generator configured toproduce first and second signals; and a first and second percutaneouselectrode array; positioning the first array on a first portion of thepatient's body and positioning the second array on a second portion ofthe patient's body such that the first and second arrays are positionedon the tissue of the patient, and the treatment site is located betweenthe first and second arrays; forming a therapeutic signal from saidfirst and second signals; and introducing the therapeutic signal throughthe first and second arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a percutaneous electrode array;

FIG. 2 is a cross-sectional view of human skin;

FIG. 3 is a side view of a percutaneous electrode array comprising anadhesion layer;

FIG. 3A is an exemplary embodiment of a percutaneous electrode arraycomprising a substrate with voids and an adhesion layer;

FIG. 3B is a top view of a percutaneous electrode array for use with anadhesion layer;

FIG. 3C is a mechanical drawing illustrating an exemplary embodiment ofa percutaneous electrode array substrate and electrodes;

FIG. 4 is a side view of an electrode substrate and an adhesion layerhaving an integrated capacitive element;

FIG. 5 is an exemplary circuit for measuring the capacitance of thecapacitive element;

FIG. 6 is a side view of an electrode comprising an integratedthermal-sensing element;

FIG. 6A is a circuit diagram of an exemplary circuit that measures thetemperature of an integrated thermistor;

FIG. 6B is a circuit diagram of an exemplary circuit that measures thetemperature of an integrated semiconductor junction;

FIG. 6C is a circuit diagram of an exemplary circuit that measures thetemperature of a thermocouple.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment disclosed provides for the application oftherapeutic electrical signals to the body through a percutaneouselectrode array. The array efficiently delivers therapeutic electricalenergy into the body provided by an electro-therapy generator device. Anelectro-therapy generator device suitable for the production of suchenergy is described in U.S. patent application Ser. No. 09/756,999,entitled “Electro-Therapy Method and Apparatus,” filed on Jan. 8, 2001,which is hereby incorporated by reference in its entirety for each ofits teachings and embodiments.

The configuration of a percutaneous electrode array is shown in FIG. 1.As shown in FIG. 1, the array comprises a substrate 110 and a pluralityof electrodes 120. Electrodes 120 are attached to a top side ofsubstrate 110. An electrical connection to the array is made on thebottom side of substrate 110 and preferably the entire bottom surface ofthe array is protected with an insulating material, for example a wovenplastic or fabric cover.

Preferably, each electrode 120 comprises a rectangular parallelepipedattached at a proximal end to the substrate. Alternatively, eachelectrode 120 preferably comprises a cylinder or cone. The distal end ofeither electrode embodiment preferably further comprises one or more ofa rounded triangular and pointed tip. The width or diameter W1 of eachelectrode is preferably between 20 to 250 micrometers.

The total surface area of the electrodes in the array equals the area ofeach electrode times the number of electrodes in contact with the skin.This area must be large enough to carry the electrical currentintroduced into the body by the electro-therapy generator device, whilelimiting the current density through the attached skin area. The surfacearea of each electrode comprises the area of the distal tip of theelectrode plus the surface area along the effective length of theelectrode, L1, i.e. the length that is inserted into the skin.Preferably, the total electrode surface area is greater than 0.2 squarecentimeters.

In an alternate preferred embodiment, the total electrode surface areais less than 0.2 square centimeters, but the substrate has a surfacearea greater than 14.1 square millimeters. The current conducting areaof the substrate in combination with the area of the electrodes limitsthe current density to the skin.

The effective contact area of the electrodes is equal to the totalsurface area of the electrodes times a 56% reduction factor thataccounts for the electrode element surface area which comes in contactwith the body's ionic environment (70% of the electrode's length), andthe number of electrodes that are in contact with the skin (80% of thetotal number of electrodes in the array). The Food and DrugAdministration (FDA) currently limits the current density forelectro-therapy devices to less than 10 milliamps per square centimeterof contact area. One with skill in the art will recognize that severaldifferent configurations can be employed in order to achieve thenecessary effective contact area needed to reduce the current densitybelow the FDA limit. One way to increase the area is to increase thelength L1 of each electrode 120 in the percutaneous electrode array,i.e., the length in contact with the ionic environment of the body, inorder to maximize the area for electrical conduction. The maximum lengthis determined by observing the structure of the skin in the human body.

FIG. 2 illustrates a typical cross section of skin. The top layer ofskin disclosed in FIG. 2, the stratum corneum, is comprised mostly ofdead skin cells. Other layers beneath the stratum corneum include thestratum lucidum, stratum granulosum, stratum spinosum and the stratumbasale. These five layers are collectively known as the epidermis. Theepidermis covers the germinating skin layers, known as the dermis, whichalso contains nerves, arteries, veins, or lymphatic vessels. Dependingon the location of the skin and its condition, the thickness of theepidermis is approximately 120 to 500 μm. The effective length ofelectrodes 120 is preferably between 120 and 500 μm, and more preferablybetween 150 and 200 μm so that the tip of electrode 120 penetrates intothe epidermis, but does not reach any nerves, arteries, veins, orlymphatic vessels. The effective length of the electrodes is preferablyadapted to the location where the array is attached and to the conditionof the skin within that region of body. The electrode length is tailoredto match these variables, enabling the electrode array to successfullytransit to a point just past the epidermis. This region is mostly devoidof pain receptors, making the insertion of the percutaneous electrodearray virtually painless. The elastic properties of the skin helps sealholes left behind by electrodes 120 after the array has been removed.Furthermore, the small diameter of each electrode 120, about thediameter of a typical human hair, will limit the amount of fluid thatcould flow through the hole created by the electrode.

The major axes of electrodes 120 are preferably perpendicular tosubstrate 110, but may be angled between perpendicular and parallel tothe substrate. Altering the mechanical properties of substrate 110and/or electrodes 120 may enhance adhesion of the array to the skin. Theelectrical contact integrity can be improved or maintained by increasingthe tension along the plane of substrate 110 between electrodes 120 andthe skin surrounding the region of penetration. For example, substrate110 may act as a spring. In this example, array 100 would be flexedprior to insertion. When array 100 is released, the tension stored insubstrate 110 would force electrodes 120 against the skin.

In an alternative preferred embodiment, array 100 comprises ashape-memory metal, e.g., Nitinol. The transition temperature of thealloy is preferably correlated with skin temperature by formulation andprocessing of the alloy. An array 100 made from such materials wouldpreferably expand or contract along a designated axis along the surfacearea of substrate 110. The expansion or contraction would forceelectrodes 120 laterally against the skin.

Electrodes 120 are preferably composed of material having goodelectrical conductive properties, such as doped silicon, silicon-metalcompounds, nickel/iron alloy, stainless steel, conductive inks, anallotrope of carbon such as glassy carbon derived from high carboncontent polymer pyrolysis, conductive polymers, polymer/graphite orpolymer/metal composite blends, and other biocompatible metals. Thematerials also have sufficient shear strength to prevent the fracture ofelectrodes in the skin. In the preferred embodiment, the array comprisestype 316 stainless steel.

As demonstrated above, the dimensions of the percutaneous electrodearray are extremely small. The development of such small structures areknown in the art as micro electrical mechanical systems, or MEMS. MEMSis a multidisciplinary field encompassing microelectronic fabrication,polymerization techniques, physical chemistry, life sciences andmechanical engineering. This cross-field environment has led to thedevelopment of micro and nano-sized structures such as micro-sensors,micro-motors and blood chemistry systems-on-a-chip. The manufacture ofsome percutaneous electrode array embodiments may draw on knowledge fromthis field, as discussed below.

In an alternative preferred embodiment, glassy carbon electrodes can bemade from any high carbon content polymer, such as pitch andpolyacrylonitrile. The material is formed into themicro-eletromechanical structures described above using the LIGAprocess. LIGA is a micromachining technology in which X-ray radiation isused in the production of high-aspect ratio, precision microstructures.LIGA parts are typically 2D extruded metal shapes, but 3D structures canbe created using this process. In the process, a master mold is createdfrom silicon using semiconductor lithographic processing. This mold isused to make replica molds by electroplating thin film silver followedby nickel. The replica mold has a thickness of 0.3 mm or greaterdepending on the mechanical loads borne by it. Next, polymeric materialis heated and softened and rolled into a film. The film is placedagainst the replica. Pressure is applied to force the polymeric materialinto the mold. After a short time period, the temperature is reduced andthe pressure removed.

Once the piece is formed, it is fired at 400 C to drive off volatilechemicals and to thermoset the plastic. This is followed by an 800 Cbake in inert atmosphere to form carbonized material. The piece isfurther baked at about 1100 C to increase conductivity by forming agraphitic phase. Due to the small size of the electrodes, the relativelylow strain properties of the material do not present a breakage problem,even after many insertion cycles.

In an alternative preferred embodiment, conductive inks are sprayed ontothe electrode array formed from a polymer such as polymethylmethacrylate, or PMMA. Moderate heating to about 120 C increases boththe conductivity and adhesion of the conductive film.

In another alternative preferred embodiment, indium tin oxide is appliedto a PMMA electrode array. A glycol-metal precursor of indium tin oxideis sprayed or spin-coated onto the array and then heated to about 400 Cto form a conductive film coating. Indium tin oxide coatings exhibitsuperb conductivity properties.

In another alternative preferred embodiment, a polymer blend is used toform the array. In such an array, a large amount of either metal powderor graphite powder or graphite-nanofiber is added to a plastic precursorto render the final material moderately conductive. Aggregation of highconcentrations of the conductive material can lead to poor uniformity inthe surface conductivity of the final composite device. Thermalprocessing of the composite, where some of the volatile components ofthe mixture are driven off, may help to reduce this deleterious effect.

In another alternative preferred embodiment, pure metal iselectrodeposited on a master mold defining the electrode structure.Preferably, the metal has a conductivity between 100 and 10000 S/cm.

In an alternate embodiment, an adhesion layer is added to the array toincrease the conductivity of the array and adhere the array to the skin.FIG. 3 illustrates a percutaneous electrode arrays that includes anadhesion layer. Array 300 comprises a substrate 310, a plurality ofelectrodes 320, and an adhesion layer 330. In a preferred method ofmanufacture, adhesion layer 330 is added to percutaneous electrode array300 by depositing material to form the layer on the surface of substrate310 between electrodes 320, or by piercing a sheet of layer materialwith array 300. Other methods may be evident to one with skill in theart.

FIGS. 3A–3C illustrate a preferred embodiment of a percutaneouselectrode array that includes an adhesion layer. More specifically,array 300 illustrated in FIG. 3A comprises a substrate 310, a pluralityof electrodes 320, an adhesion layer 330, and a plurality of voids 340in substrate 310. Adhesion layer 330 is mounted to a rear side ofsubstrate 310 and protrudes through voids 340 in substrate 310. Adhesionlayer 330 secures the electrode to the patient, and preferably aids inthe conduction of the electrical signal into the body. Substrate 310provides support for adhesion layer 330.

FIG. 3B depicts a top view of array 300 before application of adhesionlayer 330. Electrodes 320 and voids 340 are arranged in a grid pattern.Preferably, array 300 is manufactured from a sheet of stainless steelstamped and/or etched to produce voids 340 and electrodes 320 within thearea of voids 340. Electrodes 320 are bended upward so that the majoraxis is in the desired direction, preferably normal to the surface ofsubstrate 310.

FIG. 3C is a mechanical drawing depicting an exemplary embodiment ofpercutaneous electrode array 300. The array comprises 3600 electrodesarranged in a regular grid pattern of 60 by 60. The width W1 of eachelectrode is approximately 200 um. A distance S1 of about 860 umseparates the electrodes. These dimensions result in a 5 cm by 5 cmarray of electrodes. Detail A shows the electrodes within the void areabefore they are bent upwards.

Suitable materials for use in layer 330 are a hydrogel or sol-gelconstruct containing an electrolyte. The minimum height of the hydrogellayer, H1, is limited by the estimated evaporation time and themechanical modulus of the gel. In a preferred embodiment, the arraycomprises a 635 um thick conductive gel, e.g. Uni-Patch type RG63B. Asthe hydrogel is exposed to the air, the water in the gel will evaporate,drying out the array and reducing the adhesive and conductive propertiesof the gel. The use of such an array would require a higher appliedvoltage. If the array is flexed or the skin/array mechanical interfaceis otherwise altered, an instantaneous drop in interfacial impedance canoccur, giving rise to an unpleasant feeling in the patient andconcentrating the current at points of good contact, raising thepossibility of a thermal burn. Adhesion layer 330 is preferably adaptedto provide an indication that the array is no longer suitable for use.

In a preferred embodiment, the hydrogel contains materials well known inthe art that, when exposed to air after the packaging materialcontaining the electrode is opened, causes the hydrogel to slowly changecolor as a function of the evaporation rate. For example, the hydrogelmay have a normally clear appearance, but would turn into a dark colorafter exposure to the atmosphere. Alternatively, the normal appearanceof the hydrogel may be colored, and after exposure the hydrogel turnsclear. Such color changes indicate that the array needs to be replacedor that the integrity of the packaging is compromised and that the arrayis no longer sterile. In an alternate preferred embodiment, after thehydrogel has come into direct contact with human skin, a chemicalreaction would occur which changes the color of the hydrogel withoutleaving any residue on the skin.

In an alternative preferred embodiment, an adhesion layer of anelectrode is monitored to determine if the array has dried out or if thetemperature is increasing by measuring the electrical capacitance of theadhesion layer. FIG. 4 discloses the components of this embodiment. Asshown in FIG. 4, the electrode comprises a substrate 410, an adhesionlayer 430 and a capacitive plate 440 covered by an insulating layer 450.Capacitive plate 440 comprises a small section of conductive material onthe bottom side of adhesion layer 430, thus forming an electricalcapacitor comprising a dielectric (adhesion layer 430) between twoconductive plates (substrate 410 and plate 440). The capacitance of thearray capacitor is a function of both temperature and moisture content.An electrical lead is connected to plate 440 for connection in amonitoring circuit. Insulating layer 450 is coated over plate 440 toprevent plate 440 from electrically contacting the patient or others.

Circuits that measure capacitance are well known in the art. Anexemplary circuit for measuring the array capacitance is illustrated inFIG. 5. Measuring system 500 comprises a pair of identical low-passfilters 510, 520, a pair of low-offset comparators 530, 540, a flip-flop550, a binary counter 560, a microcontroller 570 and a high frequencyclock 580. A stable sinusoidal signal, a component of the signalgenerated by the electro-therapy generator device described in moredetail in U.S. patent application Ser. No. 09/756,999, entitled“Electro-Therapy Method and Apparatus,” filed on Jan. 8, 2001, is usedto determine the capacitance of adhesion layer 430.

Substrate 410 and capacitive plate 440 are connected to a monitoringcircuit comprising low-pass filters 510, 520. Filters 510, 520preferably comprise 8-pole switched capacitor filters that pass a stablesinusoidal signal. Comparators 530, 540, detect the zero crossings ofthe stable sinusoidal output applied to the reference, a fixed precisionresistor, and the array capacitor. Reference comparator 530 setsflip-flop 550, which starts counter 560, and capacitance comparator 540resets flip-flop 550, which stops counter 560. High frequency clock 580provides a clocking signal to counter 560 which increments the counteronce it is started. Counter 560 counts until the capacitance signalperforms its zero crossing. Microcontroller 570 reads the count and thenresets counter 560. Thus, counter 560 measures the time differencebetween the zero crossings of the reference signal and the currentthrough the capacitor. Microcontroller 570 determines the phase shiftbetween the signals from the count, which is indicative of thecapacitance of the array capacitor. This measurement is independent ofthe amplitude of the two signals. Microcontroller 570 comprises embeddedsoftware that uses this information to determine if the change incapacitance represents a fault state. If such a determination is made,it can shut the system down and inform the user of the error condition.The software requires that a specific profile of the change incapacitance be maintained during system operation.

FIG. 6 discloses a preferred embodiment of an electrode comprising asubstrate 610 and a temperature-sensing element 640 bonded to substrate610. The element comprises one of a thermistor, a diode, or othersemiconductor junction, and a thermocouple. In a preferred embodiment,temperature element 640 is a small device, typically no more than 0.5 mmin thickness. Temperature element 640 accurately measures thetemperature of substrate 610.

In an alternative preferred embodiment, an electrode comprising anadhesion layer has temperature-sensing element 640 embedded in theadhesion layer to monitor the integrity of the adhesion layer, forreasons stated above in the capacitance embodiment.

FIG. 6A is a circuit diagram of an exemplary circuit that measures thetemperature of a percutaneous electrode array comprising an integratedthermistor. In this embodiment, thermistor element 640 is connected to amonitoring circuit comprising a voltage divider bridge circuit 650, adifferential amplifier 660, an analog-to-digital converter 670 and amicrocontroller 680. Amplifier 660 eliminates any common mode noiseassociated with the lead length from the element 640 to the monitoringcircuit. The resultant voltage from amplifier 660 varies as a functionof array temperature. The monitoring circuit converts the voltage signalto a binary value by analog-to-digital converter 670. The monitoringcircuit further comprises microcontroller 680 having software thatconverts the binary representation of the voltage signal into thetemperature of the array.

FIG. 6B is a circuit diagram of an exemplary circuit that measures thetemperature of a percutaneous electrode array comprising either anintegrated semiconductor or discrete-device semiconductor junction. Inthis embodiment, element 640 comprises a diode or transistor having awell-characterized, temperature dependent behavior that measurestemperature to a high precision. As shown in FIG. 6B, the junction isconnected to a monitoring circuit comprising a constant current source651, a reference resistor 652, an amplifier 660, an analog-to-digitalconverter 670, and a microcontroller 680. The current is supplied tojunction 640 through resistor 652 to forward bias junction 640. Avoltage is measured across junction 640, which varies with junctiontemperature. The relationship between the junction voltage andtemperature is:

Vjunction=kT/q*ln(Ijunction/Ijunction saturation current), where k isBoltzmann's constant (1.38×10⁻²³ J/K), T is the absolute temperature indegrees Kelvin, q is the electron charge (1.601×10⁻¹⁹ coulomb),Ijunction is the constant supplied reference current, and Ijunctionsaturation current is the saturation current of the semiconductor device(2×10⁻¹⁶ A for silicon).

Amplifier 660 increases the junction voltage to a useful level andconverter 670 transforms the signal into a binary representation.Microcontroller 680 uses the binary representation to determine thearray temperature.

FIG. 6C is a circuit diagram of an exemplary circuit that measures thetemperature of a percutaneous electrode array comprising a thermocouple.In this embodiment, temperature element 640 comprises a thermocouple. Athermocouple is a device comprising two dissimilar metals (e.g.,platinum and rhodium) in electrical contact with each other at ajunction. The device generates an electromotive force correlated to thetemperature at the junction. The thermocouple requires compensation forthe temperature of the junctions formed between the device and itsconnecting leads (cold-junction compensation).

The monitoring circuit illustrated in FIG. 6C comprises an amplifier660, a analog-to-digital converter 670 and a microcontroller. Amplifier660 amplifies thermocouple 640's output voltage, converter 670 convertsit to a binary representation, and then software in microcontroller 680uses the binary voltage value to determine the array's temperature.Amplifier 660 contains the necessary components to effect cold-junctioncompensation circuitry as is well known in the art. The softwarecontains a lookup table as is well known in the art to convert thebinary representation of thermocouple voltage to temperature.

In a preferred embodiment, the measured temperature parameter is used asan interlock in the electro-therapy generator device to protect thepatient from harm. If for some reason the array rises above 40 degreesCelsius, or ramps up in temperature at a higher rate than would normallybe expected, a temperature-monitoring portion of the electro-therapygenerator device can interrupt its output, thus lessening or eliminatingthe possibility of a bum or thermal irritation. Such detected conditionsare used to inform the operator of potential problems with the integrityof the percutaneous electrode array, or the adhesion or placement of thearray, two of the most likely causes of an increase in current density.

In another embodiment, the electro-therapy generating devicecontinuously monitors the impedance of the percutaneous electrode array.The device includes a warning indicator which alerts the operator whenthe impedance of the percutaneous electrode array is too high,indicating that the array should be checked or replaced. The indicatorwould provide one or more of a visual indication, for example a blinkinglight emitting diode (LED) or an error message on an liquid crystaldisplay (LCD), an audio indication such as a beeping sound, and asensory indication such as a vibration producing device. The warningindicator can also be used to indicate error conditions such as a loosearray, unplugged lead wires, weak batteries, missing temperature signal,missing capacitance monitoring signal, or any other defective conditionof the array.

While the invention has been described with reference to preferredembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A percutaneous electrode array for delivering therapeutic electricalenergy to a patient, comprising: an electrically conductive substratehaving a top side and a bottom side; and a plurality of electrodes eachhaving a proximal end, a distal end, an axis from said proximal end tosaid distal end, and a length along said axis, wherein each electrode isattached to said top side of said substrate; wherein said electrodeshave a total surface area of more than 0.2 square centimeters; andwherein said electrodes are electrically connected together by saidsubstrate.
 2. The array of claim 1, wherein the electrodes aresubstantially a cylinder and have a diameter of 20 to 250 micrometers.3. The array of claim 1, wherein the electrodes are substantially arectangular parallelepiped having a pair of narrow sides, a pair of widesides, a top side and a bottom side, and wherein the wide sides have awidth of 20 to 250 micrometers.
 4. The array of claim 3, wherein thewide sides have a width of about 200 micrometers.
 5. The array of claim1, wherein the length of the electrodes is between 120 and 500micrometers.
 6. The array of claim 1, wherein the length of theelectrodes is between 150 and 200 micrometers.
 7. The array of claim 1,wherein the distal end of each electrode is one or more of thinned andpointed to facilitate placement into skin.
 8. The array of claim 1,wherein the axis of the electrodes is perpendicular to the substrate. 9.The array of claim 1, wherein the axis of the electrodes is angledbetween perpendicular and parallel to the substrate.
 10. The array ofclaim 1, wherein the substrate comprises a shape-memory metal alloy. 11.The array of claim 1, wherein the electrodes comprise one or more ofdoped semiconductor material, silicon-metal compound, stainless steel,conductive polymer, carbon allotrope, and a conductive metal either inbulk or deposited material.
 12. The array of claim 1, further comprisinga temperature element bonded to the substrate.
 13. The array of claim12, wherein the temperature element is one of a thermistor, a diode, asemiconductor junction, and a thermocouple.
 14. The array of claim 1,further comprising an adhesion layer.
 15. The array of claim 1, furthercomprising: a plurality of voids that pass through the top side of thesubstrate to the bottom side; and an adhesion layer comprising a bottomside, a top side, and a plurality of protrusions extending above the topside; wherein the top side of the adhesion layer is attached to thebottom side of the substrate, and the protrusions pass through the voidsto a first height above the top side of the substrate.
 16. The array ofclaim 15, wherein the electrodes extend above the first height of theadhesion layer between 150 and 200 micrometers.
 17. The array of claim15, wherein the electrodes have a total surface area above the firstheight of the adhesion layer of at least 0.2 square centimeters.
 18. Thearray of claim 15, wherein the adhesion layer comprises an electricallyconductive hydrogel.
 19. The array of claim 15, wherein the adhesionlayer comprises a removable medical adhesive.
 20. The array of claim 15,wherein the adhesion layer changes color as a function of ambientconditions.
 21. The array of claim 15, further comprising a capacitiveplate disposed on the bottom side of the adhesion layer and anelectrically insulating layer disposed on the capacitive plate oppositethe adhesion layer.
 22. The array of claim 21, further comprising atemperature element embedded in the adhesion layer.
 23. A percutaneouselectrode array for delivering therapeutic electrical energy to apatient, comprising: an electrically conductive substrate having a topside and a bottom side; and a plurality of electrodes each having aproximal end, a distal end, an axis from said proximal end to saiddistal end, and a length along said axis; wherein each electrode isattached to said top side of said substrate, said substrate has asurface area greater than 14.1 square millimeters and said electrodeshave a total surface area of less than 0.2 square centimeters; andwherein said electrodes are electrically connected together by saidsubstrate.
 24. An electrode for delivering therapeutic electrical energyto a patient, comprising: a substrate having a first side and a secondside; an adhesion layer comprising a bottom side and a top side attachedto the first side of the substrate; a capacitive plate disposed on thebottom side of the adhesion layer; and an electrically insulating layerdisposed on the capacitive plate opposite the adhesion layer.
 25. Thearray of claim 1, wherein a temperature element is bonded to saidsubstrate.